Lithium metal negative electrode and lithium metal battery including the same

ABSTRACT

A lithium metal negative electrode and a lithium metal battery including the lithium metal negative electrode. The lithium metal negative electrode includes a protective layer present on at least one surface of the negative electrode for stabilizing between the lithium metal and the electrolyte. The protective layer includes a polymer of alpha lipoic acid (ALA) and sulfur molecule (S 8 ), a depolymerized product of the polymer, an inorganic sulfide-based compound, and at least one of an inorganic nitride-based compound, or an inorganic nitrate-based compound.

CROSS-CITATION WITH RELATED APPLICATION(S)

This application claims the benefit of Korean Patent Application No.10-2019-0085902 filed on Jul. 16, 2019 with the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a lithium metal negative electrode,and a lithium metal battery including the same

BACKGROUND ART

Lithium metal battery is a secondary battery using lithium metal(Li-metal) as a negative electrode active material, and can exhibit ahigh theoretical capacity (3860 mAh/g) and a low standard reductionpotential (−3.040 V vs. SHE). Thus, the lithium metal battery isspotlighted as a next-generation battery for replacing lithium-ionbatteries that use carbon-based anode active materials such as graphite.

However, due to the high reactivity of the lithium metal, the fact thatthe coulombic efficiency is low, the lifespan is short, and the safetyis low as compared with the lithium ion battery is a factor that delaysthe commercialization of the lithium metal battery.

Specifically, during driving of the lithium metal battery, dendriticlithium dendrite and dead lithium may be formed on the surface of thelithium metal negative electrode. These materials can form a solidelectrolyte interphase (SEI) on the surface of a lithium metal negativeelectrode, and the process of being easily broken and formed can berepeated.

Accordingly, the negative electrode active material capable ofparticipating in the electrochemical reaction is gradually lost, wherebythe coulombic efficiency decreases and the lifetime of the lithium metalbattery may be shortened.

Moreover, lithium dendrites grown from the surface of a lithium metalnegative electrode penetrate the separator and reach the positiveelectrode, and may cause an internal short circuit of the lithium metalbattery, which is directly related to safety problems such as fire andexplosion.

Therefore, in order to implement a lithium metal battery that ensuressafety while exhibiting high performance, it is essential to suppressthe growth of dendrites on the surface of the lithium metal negativeelectrode and improve the reversibility of the electrodeposition anddesorption reaction of the lithium metal on the surface of the lithiummetal negative electrode.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

In one embodiment of the present disclosure, a stable protective layeris formed on one surface or both surfaces of the lithium metal negativeelectrode, so as to suppress the growth of dendrites on the surface.

Further, the electrodeposition and desorption reaction of lithium metalin the lower part of the protective layer is reversibly performed withexcellent efficiency, and ultimately, thereby specifying the componentsof the protective layer and the method of forming the protective layerso that the lifetime of the lithium metal battery can be improved.

Technical Solution

Throughout the specification, when a part is referred to as “including”a certain component, it means that it can further include othercomponents, without excluding the other components, unless otherwisestated. The term “about or approximately” or “substantially” used hereinis intended to have meanings close to numerical values or rangesspecified with an allowable error and intended to prevent accurate orabsolute numerical values disclosed for understanding of the presentdisclosure from being illegally or unfairly used by any unconscionablethird party. Further, throughout the specification, the term “step of”does not mean “step for”.

Throughout the specification, the term “combination(s) thereof” includedin Markush type description means mixture or combination of one or moreselected from a group consisting of components described in Markush typeand thereby means including one or more selected from a group consistingof the components.

Based on the above definitions, embodiments of the present disclosurewill be described in detail. However, these are presented forillustrative purposes only, and the present disclosure is not limitedthereto, and the present disclosure is only defined by the scope of theclaims described later.

A lithium metal negative electrode according to one embodiment of thepresent disclosure includes a negative electrode and a protective layerpresent on at least one surface of the negative electrode, wherein theprotective layer includes all of the following (A) to (D) as componentsthereof:

(A) a polymer of alpha lipoic acid (ALA) and sulfur molecule (S₈);

(B) a depolymerized product of the polymer;

(C) an inorganic sulfide-based compound; and

(D) an inorganic nitride-based compound, an inorganic nitrate-basedcompound, or a mixture thereof.

As used herein, the “protective layer” is defined as a concept thatincludes not only when the (A) to (D) are directly coated onto thesurface of the lithium metal layer, but also when a solid electrolyteinterface (SEI) that is present in the space between an electrolyte anda lithium metal layer in the battery containing the lithium metalnegative electrode of the one embodiment, and thus stabilizes theinterface.

The protective layer of the one embodiment may be formed by an ex-situmethod, but may be formed by an in-situ method. In the latter case, astable interface can be formed while minimizing the resistance of thelithium metal battery.

In another embodiment of the present disclosure, there is provided amethod of forming the protective layer of the one embodiment in anin-situ manner in the process of producing a lithium metal battery.Specifically, a protective layer including all of the above (A) to (D)may be formed in a series of manufacturing processes of a lithium metalbattery, including the steps of: preparing a polymer of alpha lipoicacid (ALA) and sulfur molecule (S₈); coating the polymer onto onesurface of the separator; preparing an electrode assembly wherein theseparator surface coated with the polymer is positioned opposite to alithium metal negative electrode, and the other surface of the separatoris positioned opposite to a negative electrode, injecting an electrolyteinto the separator in the electrode assembly; and packaging the assemblyafter injecting the electrolyte to obtain a lithium metal battery.

In the case of the negative electrode to be formed of the protectivelayer, it may be a lithium-free negative electrode (Li free anode)composed of only a copper current collector; and may include a coppercurrent collector; and a lithium metal layer present on the coppercurrent collector. In the former case, when the battery is assembled, alithium free anode composed of only a copper current collector may beformed, but a lithium metal layer may be formed on the surface of thecopper current collector in accordance with battery charge/discharge.

Thus, in the obtained lithium metal battery, a part of the polymercoated onto the surface of the separator may be depolymerized byreacting the polymer with the lithium metal of the negative electrode,another part of the polymer coated onto the surface of the separator andthe depolymerized product may be reduced and decomposed by reacting withthe lithium metal of the negative electrode, and wherein the electrolytecomprises LiNO₃ and the LiNO₃ may be reduced and decomposed by reactingwith the lithium metal of the negative electrode.

In another embodiment of the present disclosure, there is provided alithium metal battery including: the above-mentioned lithium metalnegative electrode; a positive electrode; a separator present betweenthe lithium metal negative electrode and the positive electrode; and anelectrolyte impregnated in the separator.

Hereinafter, the components of the protective layer will be describedwith reference to FIG. 1, and an in-situ formation method of theprotective layer including the components will be described.

(A) Polymer of Alpha Lipoic Acid (ALA) and Sulfur Molecule (S₈)

The alpha lipoic acid (ALA) is known to have a melting point of 63° C.,and when heated at a temperature higher than that, due to thecharacteristics of the unstable disulfide bond (S—S) in the molecule andthe spatial stress of the pentagonal ring, the ring is opened to form aradical in sulfur at the terminal.

When alpha lipoic acid (ALA) is present alone and is heated above itsmelting point, the above-mentioned ring opening reaction and radicalpolymerization reaction occur, so that poly alpha lipoic acid(poly(ALA)) can be synthesized.

Meanwhile, the sulfur molecule S₈ is an octagonal ring-shaped molecule,and may be ring-opened at a temperature of about 159° C. or more to forma radical. In this regard, when the mixture of sulfur molecules (S₈) andalpha lipoic acid (ALA) is heated above the ring opening temperature ofthe sulfur molecules (S₈), each ring is opened, and radicalpolymerization can be proceeded.

The resulting polymer of sulfur molecules (S₈) and alpha lipoic acid(ALA) (hereinafter, also referred to as poly(ALA-S) in some cases) canbe represented by the following Chemical Formula 1:

in the Chemical Formula 1, R₁ to R₄ may be each independently asubstituted or unsubstituted C1 to C3 alkylene group, specifically anunsubstituted C1 alkylene group; may be 3≤n≤10, specifically 2≤n≤8; andmay be 1<x≤10, specifically 2<x≤9.

The structure of Chemical Formula 1 may be related to the synthesisprocess of poly(ALA-S). Specifically, the sulfur molecule (S₈) may bering-opened to form a radical, and then bonded to the ring-openedterminal (i.e., sulfur chain) of the alpha lipoic acid (ALA).

Meanwhile, in the synthesis of poly(ALA-S), the weight ratio of alphalipoic acid (ALA) and sulfur molecule (S₈) is not particularly limited,but may be in the range of 8:2 to 3:7, specifically 5:5 to 3:7. Thehigher the content of sulfur molecules (S₈) within this range, the lowerthe ionic conductivity of the lithium metal battery, while the reductiondecomposition proceeds smoothly as will be described later to exhibitthe effect of forming an inorganic sulfide-based compound.

According to the ionic conductivity experiment (Experimental Example 2)described later, the ionic conductivity tends to decrease as the sulfurcontent increases, but even in the case of 3:7, which is the highestsulfur content, it exhibits a level of 10⁻⁵ S/cm where ionic conductionis sufficiently possible. In contrast, the poly(ALA) does not readilyundergo reductive decomposition on the metal surface. As the sulfurcontent increases, the reductive decomposition of poly(ALA-S) proceedssmoothly, so that more inorganic sulfide-based SEI (Li₂S and Li₂S₂) isproduced on the metal surface, and as the sulfur content is higherduring actual battery operation, the overvoltage may be rather lowerduring actual cell operation.

(B) Depolymerized Product of the Polymer

As described above, the protective layer of the one embodiment can beformed by an in-situ method. To this end, a lithium metal battery can bemanufactured in a series of steps of coating the poly(ALA-S) onto onesurface of the protective film, and then forming an assembly of anegative electrode-separator-positive electrode structure so as to beopposed to the lithium metal of the negative electrode including thenegative electrode current collector and the lithium metal layer locatedon the surface, and injecting an electrolyte.

In the lithium metal battery manufactured thereby, the lithium metallayer of the negative electrode and the polymer coated on one surface ofthe separator may come into contact with each other.

When the (A) poly(ALA-S) comes into contact with the lithium metallayer, it may be depolymerized to form the (B) oligomer, monomer, or amixture thereof. Since the potential of this reaction is higher than thepotential at which the reductive decomposition of electrolyte occurs, ittakes precedence over electrolyte decomposition due to the reactionbetween the electrolyte and lithium metal.

First, the poly(ALA-S) may be depolymerized by reacting with the lithiummetal on the surface of the lithium metal to form an oligomerrepresented by the following Chemical Formula 1-1:

in the Chemical Formula 1-1, the R₁ to R₄ may be each independently asubstituted or unsubstituted C1 to C3 alkylene group, specifically anunsubstituted C1 alkylene group; may be 1≤y≤6, specifically 1≤y≤5; andmay be 1≤z≤6, specifically 1≤z≤5.

The oligomer represented by Chemical Formula 1-1 is a type of thiolate,and may form a protective layer on the surface of the lithium metalnegative electrode together with lithium cations (Lit) dissociated fromthe additive. Hereinafter, in some cases, the compound of oligomer andlithium cation represented by Chemical Formula 1-1 are also referred toas Li-poly(ALA-S) thiolate.

The oligomer represented by Chemical Formula 1-1 is reduced by a lowreduction potential of lithium, and a reaction in which the terminalsulfur chain is further broken may proceed to form a monomer representedby Chemical Formula 1-2:

in the Chemical Formula 1-2, R₁ to R₄ may be each independently asubstituted or unsubstituted C1 to C3 alkylene group, specifically anunsubstituted C1 alkylene group.

The monomer represented by Chemical Formula 1-2 is also a type ofthiolate, and can form a protective layer on the surface of the lithiummetal negative electrode together with the lithium cation (Li⁺)dissociated from the additive. Hereinafter, in some cases, the compoundof the monomer and lithium cation represented by Chemical Formula 1-2 isalso referred to as Li-ALA thiolate.

Considering the formation process of the oligomer represented byChemical Formula 1-1 and the monomer represented by Chemical Formula1-2, it can be seen that the composition in the final protective layermay vary depending on whether the poly(ALA-S) in the protective layer ispartially or completely depolymerized.

Specifically, the protective layer of the one embodiment may include thepoly(ALA-S) alone, but further include an oligomer, a monomer or amixture thereof by depolymerization of the poly (ALA-S), together withthe poly(ALA-S). Alternatively, it is possible to include an oligomer, amonomer, or a mixture thereof obtained by depolymerization of thepoly(ALA-S), without including the poly(ALA-S).

(C) Inorganic Sulfide Compound

On the other hand, when the driving voltage of the lithium metal batterydrops below 0V, since the voltage is lower than the equilibriumpotential of the oxidation-reduction reaction of sulfur or the sulfurfunctional group, it can be electrochemically reduced and decomposed toproduce the (C) inorganic sulfide-based compound.

The poly(ALA-S) is easily decomposed chemically or electrochemically ina state of being in contact with the lithium metal layer. The (A)poly(ALA-S) forms a uniform interface with lithium metal which also hasa high surface energy, due to the high surface energy of the lithiummetal layer. At the interface thus generated, a chemical decompositionreaction occurs due to a difference in potential between poly(ALA-S)containing a large amount of sulfur functional groups (2.1 to 2.3V vsSHE) having a high reduction potential and lithium metal having a lowreduction potential (−3.04V vs SHE).

As a result of the decomposition, an inorganic sulfide-based compoundincluding Li₂S, Li₂S₂, or a mixture thereof may be formed. Such adecomposition reaction can easily occur compared to poly(ALA), due tothe structure of poly(ALA-S) represented by Chemical Formula 1.

In this regard, among the components of the protective layer of the oneembodiment, the (C) inorganic sulfide-based compound may be a product inwhich the (A) poly (ALA-S) is chemically or electrochemically decomposedin contact with the lithium metal layer. The (C) inorganic sulfide-basedcompound may contribute to stabilizing an interface with an electrolytein a lithium metal battery.

(D) Inorganic Nitride-Based Compound, Inorganic Nitrate-Based Compound,or Mixture Thereof

The (C) inorganic sulfide-based compound is stable because it canwithstand a rapid volume expansion of the lithium metal layer and/or thelithium metal battery, but has a disadvantage that the ionicconductivity is lowered.

In this regard, in the one embodiment, LiNO₃ is added to the electrolyteso that the LiNO₃ reacts with the lithium metal layer of the negativeelectrode to be reduced and decomposed, and an inorganic nitride-basedcompound, an inorganic nitrate-based compound, or a mixture thereof,which is the reductive decomposition product (D), is also made to be thecomponent of the protective layer.

Specifically, a single material of the inorganic nitride-based compoundor the inorganic nitrate-based compound may be produced as a reductionand decomposition product by the reaction of the lithium metal layer ofthe negative electrode and the LiNO₃, but a mixture of the two materialsmay be produced.

Here, the inorganic nitride-based compound may include Li₃N, and theinorganic nitrate-based compound may include LiN_(x)O_(y) (where x:y=1:2to 1:3). These have higher ionic conductivity than the (C) inorganicsulfide-based compound, and thus may contribute to resistance reductionof the negative electrode of the one embodiment and improvement of theionic conductivity.

Method for Manufacturing a Lithium Metal Battery (In-Situ FormationMethod of the Protective Layer)

The protective layer including all of the above (A) to (D) may be formedby an in-situ method during the manufacturing process of a lithium metalbattery, as previously mentioned.

As previously mentioned, the protective layer including all of the above(A) to (D) may be formed in a series of manufacturing processes of alithium metal battery, including the steps of: preparing a polymer ofalpha lipoic acid (ALA) and sulfur molecule (S₈); coating the polymeronto one surface of the separator; preparing an electrode assembly so asto be opposed to one surface of a separator coated with the polymer to alithium metal negative electrode, and be opposed to the other surface ofthe separator to a positive electrode, thereby preparing an electrodeassembly, injecting an electrolyte into the separator in the assembly;and packaging the assembly into which the electrolyte is injected,thereby obtaining a lithium metal battery.

More specifically, in the lithium metal battery obtained according tothe method of the one embodiment, a part of the polymer coated on thesurface of the separator may be depolymerized by reacting with thelithium metal of the negative electrode to form the (B); another part ofthe polymer coated on the surface of the separator may be reduced anddecomposed by reacting with the electrolyte to form the (C); LiNO₃ addedto the electrolyte may be decomposed and reduced by reacting with thelithium metal of the negative electrode to form (D).

The step of preparing the polymer may be a step of heat-treating amixture of alpha lipoic acid (ALA) and sulfur molecule (S₈) as describedabove to form a radical by ring opening and proceeding a radicalpolymerization reaction.

In this regard, the heat treatment temperature may be a higher ringopening temperature among the alpha lipoic acid (ALA) and sulfurmolecules (S₈), that is, 159° C. or more, which is the ring openingtemperature of the sulfur molecules (S₈), and the temperature is notparticularly limited.

Experimentally, the heat treatment temperature can be controlled from150° C. or more to 180° C. or less, specifically from 159° C. or more to175° C. or less, during the step of preparing the polymer, and in thisrange, the process efficiency may be excellent, and the thermal damageof the synthesized polymer may be small.

In the step of coating the polymer onto one surface of the separator,the coating method is not particularly limited, and an appropriatemethod may be selected from methods well known in the art, such as spraycoating and dip coating.

Since the protective layer is formed after being coated onto theseparator, the thickness of the (A) poly(ALA-S) coating onto theseparator can be discussed. Although not particularly limited, thethickness of the (A) poly(ALA-S) coating onto the separator to form theprotective layer may be 1 μm to 10 μm, specifically 1 μm to 3 μm, and inthis range, it exhibits the effect of stabilizing the interface betweenthe lithium metal layer and the electrolyte.

In addition, between the lithium metal negative electrode and theprotective layer, a polymer (poly(ALA-S)) in the protective layer can bedepolymerized to form the oligomer represented by Chemical Formula 1-1,the monomer represented by Chemical Formula 1-2, or mixtures thereof.These can also be the components of the protective layer as describedabove.

The electrolyte for accelerating the depolymerization may include alithium salt, an organic solvent, and LiNO₃ as an additive. Thefunctions and effects of the additive LiNO₃ are as described above.

The concentration of the additive in the electrolyte may be 0.1 to 1.0M, specifically 0.2 to 0.7 M, for example 0.5 to 0.7 M. As theconcentration increases in this range, the lifetime of the lithium metalbattery can be increased by a synergistic effect of the (C) inorganicsulfide-based compound, the (D) inorganic nitride-based compound, theinorganic nitrate-based compound, or a mixture thereof.

The lithium salt is not particularly limited, but may include lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI). This has a high solubilityin an ether-based solvent compared to other lithium salts such as LiPF₆,and has an effect of forming LiF, which is known to have an effect ofstabilizing the surface of lithium metal during reduction decomposition.

The concentration of the lithium salt is also not limited, but may becontrolled within the range of 0.1 to 5.0M. In this range, theelectrolyte may have an appropriate conductivity and viscosity, andlithium ions may effectively move within the lithium metal battery ofthe one embodiment. However, this is only an example, and the presentdisclosure is not limited thereby.

The organic solvent may be a mixture of 1,3-dioxolane (DOL) anddimethoxy ethane (DME) in a ratio of 3:7 to 7:3, specifically 5:5 to 7:3by volume. This is more effective than other solvents such ascarbonate-based solvents.

Meanwhile, even in the lithium metal battery finally obtained accordingto the one embodiment, at least one surface of the separator opposing tothe negative electrode may contain a coating layer including thepolymer, which may mean that the polymer coated onto the surface of theseparator before assembling the battery is not completely dissolved inthe electrolyte after assembling the battery, and it is coated on theseparator again after being dissolved.

In the one embodiment, the separator is not particularly limited, andmay be in the form of a porous film. Specifically, the separatorincludes any one selected from the group consisting of polyolefin,polyester, polysulfone, polyimide, polyetherimide, polyamide,polytetrafluoroethylene, rayon, glass fiber, and mixtures thereof, or itmay be a multilayer film thereof. More specifically, the porousseparator may be a porous polymer film made of a polyolefin-basedpolymer such as an ethylene homopolymer, a propylene homopolymer, anethylene/butene copolymer, an ethylene/hexene copolymer and anethylene/methacrylic acid copolymer, or a laminated structure having twoor more layers thereof.

Further, the separator may have a porosity of 20 to 80% by volume withrespect to the total volume of the separator, in a state in which afluoro-ionomer is not filled. If the porosity of the separator is lessthan 20% by volume, there may be a problem that the pores decreaserapidly, Li ion transfer becomes difficult, and thus the resistance ofthe separator increases. If the porosity of the separator exceeds 80% byvolume, the mechanical strength of the separator is reduced, which maycause a problem of tearing during cell assembling.

Further, the separator may include a first pore having a D50 of 1 nm to200 nm in the separator. If the D50 of the first pore is less than 1 nm,the coating layer is not easily formed, or the improvement effect due tothe formation of the coating layer is insignificant, and if the D50 ofthe first pore exceeds 200 nm, there is a risk that the mechanicalstrength of the separator itself is reduced.

Meanwhile, the separator may be a so-called safety reinforced separator(SRS) whose surface is coated with inorganic particles to enhancethermal stability, mechanical stability, and the like.

The inorganic particles may be coated via a binder, and here, as theinorganic particles and the binder, those commonly known in the art canbe used.

In the Experimental Examples described later, both of the two electrodeswere made of lithium metal. However, this is due to experimentalconvenience, and the lithium metal battery of the one embodiment maygenerally use a positive electrode including a positive electrodecurrent collector and a positive electrode mixture layer present on thepositive electrode current collector, as is known in the art.

The positive electrode is manufactured by mixing an active material anda binder, optionally a conductive material, a filler, and the like in asolvent to produce an electrode mixture slurry, and then coating thiselectrode mixture slurry onto each positive electrode current collector.Since the above-mentioned electrode-manufacturing method is widely knownin the art, a detailed description thereof will be omitted herein.

In the case of the positive electrode active material, there is noparticular limitation as long as it is a material capable of reversiblyintercalating and de-intercalating lithium ions. For example, it mayinclude one or more of complex oxides of cobalt, manganese, nickel, or acombination of metals; and lithium.

In a more specific example, a compound represented by any of thefollowing chemical formulas can be used as the positive electrode activematerial. Li_(a)A_(1-b)R_(b)D₂ (wherein, 0.90≤a≤1.8 and 0≤b≤0.5);Li_(a)E_(1-b)R_(b)O_(2-c)D_(c) (wherein, 0.90≤a≤1.8, 0≤b≤0.5, and0≤c≤0.05); LiE_(2-b)R_(b)O_(4-c)D_(c) (wherein, 0≤b≤0.5, and 0≤c≤0.05);Li_(a)Ni_(1-b-c)Co_(b)R_(c)D_(α) (wherein, 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05and 0<α≤2); Li_(a)Ni_(1-b-c)Co_(b)R_(c)O_(2-α)Z_(α) (wherein,0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05 and 0<α<2);Li_(a)Ni_(1-b-c)Co_(b)R_(c)O_(2-α)Z₂ (wherein, 0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05 and 0<α<2); Li_(a)Ni_(1-b-c)Mn_(b)R_(c)D_(α) (wherein,0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05 and 0<α≤2);Li_(a)Ni_(1-b-c)Mn_(b)R_(c)O_(2-α)Z_(α) (wherein, 0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05 and 0<α<2); Li_(a)Ni_(1-b-c)Mn_(b)R_(c)O_(2-α)Z₂ (wherein,0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05 and 0<α<2); Li_(a)Ni_(b)E_(c)G_(d)O₂(wherein, 0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5 and 0.001≤d≤0.1);Li_(a)Ni_(b)Co_(c)Mn_(d)G_(e)O₂ (wherein, 0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5,0≤d≤0.5 and 0≤e≤0.1); Li_(a)NiG_(b)O₂ (wherein, 0.90≤a≤1.8 and0.001≤b≤0.1); Li_(a)CoG_(b)O₂ (wherein, 0.90≤a≤1.8 and 0.001≤b≤0.1);Li_(a)MnG_(b)O₂ (wherein, 0.90≤a≤1.8 and 0.001≤b≤0.1); Li_(a)Mn₂G_(b)O₄(wherein, 0.90≤a≤1.8 and 0.001≤b≤0.1); QO₂; QS₂; LiQS₂; V₂O₅; LiV₂O₅;LiTO₂; LiNiVO₄; Li_((3-f))J₂(PO₄)₃ (0≤f≤2); Li_((3-f))Fe₂(PO₄)₃ (0≤f≤2);and LiFePO₄.

In the above chemical formulas, A is Ni, Co, Mn or a combinationthereof; R is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element ora combination thereof; D is O, F, S, P or a combination thereof; E isCo, Mn or a combination thereof; Z is F, S, P or a combination thereof;G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V or a combination thereof; Q isTi, Mo, Mn or a combination thereof; T is Cr, V, Fe, Sc, Y or acombination thereof; and J is V, Cr, Mn, Co, Ni, Cu or a combinationthereof.

Of course, it is also possible to use one having a coating layer on thesurface of the above-mentioned compound, or it is possible to use amixture of the above-mentioned compound with a compound having a coatinglayer. The coating layer may include a coating element compound such ascoating element oxide, hydroxide, coating element oxyhydroxide, coatingelement oxycarbonate or coating element hydroxycarbonate. The compoundsforming these coating layers may be amorphous or crystalline. As acoating element included in the coating layer, Mg, Al, Co, K, Na, Ca,Si, Ti, V, Sn, Ge, Ga, B, As, Zr or a mixture thereof can be used. Asthe coating layer forming process, any coating method can be used aslong as it can be coated by a method (e.g., spray coating or dippingmethod, etc.) that does not adversely affect the physical properties ofthe positive electrode active material by using these elements in thecompound. Since this is a content that may be widely understood by thoseworked in the art, and thus, detailed descriptions thereof will beomitted.

The positive electrode current collector is typically fabricated to athickness of 3 to 500 μm. The positive electrode current collector isnot particularly limited as long as a corresponding battery has highconductivity without causing a chemical change in the battery, and forexample, may be formed of stainless steel, aluminum, nickel, titanium,baked carbon, or aluminum, or a material formed by surface-treating asurface of stainless steel with carbon, nickel, titanium, silver, or thelike. The current collector may have fine protrusions and depressionsformed on a surface thereof to enhance adherence of a positive electrodeactive material, and may be formed in various forms such as a film, asheet, a foil, a net, a porous body, a foaming body, and a non-wovenfabric structure.

The conductive material is not particularly limited as long as acorresponding battery has high conductivity without causing a chemicalchange in the battery, and for example, graphite such as naturalgraphite and artificial graphite; carbon blacks such as carbon black,acetylene black, ketjen black, channel black, furnace black, lamp black,and thermal black; conductive fibers such as carbon fiber and metalfiber; carbon fluoride powder; metal powders such as aluminum powder,and nickel powder; conductive whiskers such as zinc oxide and potassiumtitanate; conductive metal oxides such as titanium oxide; conductivematerials such as polyphenylene derivatives may be used.

The lithium metal battery of the one embodiment may not only be used ina unit cell used as a power source for a small device, but also it canbe used as a unit cell in a medium or large-sized battery moduleincluding a plurality of battery cells. Furthermore, a battery packincluding the battery module may be configured.

Advantageous Effects

The lithium metal negative electrode of the one embodiment can suppressthe growth of dendrites by a stable protective layer on one surface orboth surfaces thereof, can reversibly perform the electrodeposition anddesorption reaction of lithium metal in the lower part of the protectivelayer with excellent efficiency, and can ultimately contribute to theimprovement of the life of the lithium metal battery.

The protective layer can be formed by an in-situ method, whereby astable interface can be formed while further minimizing the resistanceof the lithium metal battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically illustrating an in-situ formationmethod of a protective layer including components of a protective layeraccording to an embodiment of the present disclosure.

FIG. 2 shows the results of XRD analysis by changing the polymer inExperimental Example 1 described later.

FIGS. 3a and 3b show the results of photographing the coated surfacewith a digital camera (FIG. 3a ) and SEM (FIG. 3b ) by changing thepolymer, which is the separator coating material, in ExperimentalExample 2 described later. FIG. 3c shows the ionic conductivity of eachcoated separator.

FIGS. 4a to 4c are the results of testing the charge/dischargeperformance of lithium symmetric cells by changing the concentration ofthe electrolyte additive (FIG. 4a ), changing the separator coatingmaterial (FIG. 4b ), and changing the type of electrolyte additive (FIG.4c ), in Experimental Example 3 described later.

FIG. 4d shows the results of photographing the surface of each lithiummetal negative electrode with a digital camera.

FIGS. 4e and 4f show the results of recovering a lithium metal negativeelectrode from each of the lithium symmetric cells evaluated in FIG. 4cto analyze by X-ray photoelectron analysis method.

FIGS. 5a and 5b show the results of testing the charge/dischargeperformance of a lithium half-cell by changing the separator coatingmaterial in Experimental Example 4 described later.

FIG. 5c shows the EIS (electrochemical impedance spectroscopy) analysis(Li/SUS) to which Poly(ALA-S) is applied in Example 3, and FIG. 5d showsthe EIS analysis for Comparative Example 1.

FIG. 6 show the results of testing the charge/discharge performance oflithium half cells by changing the separator coating material, andchanging whether or not the electrolyte additive is applied, inExperimental Example 5 described later.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, preferred examples of the present disclosure, comparativeexamples, and test examples for evaluating them are described. However,the following examples are only preferred examples of the presentdisclosure, and the present disclosure is not limited to the followingexamples.

Examples 1 to 3

(1) Preparation of Poly(ALA-S)

Each mixture in which the weight ratio of alpha lipoic acid (ALA) powderand sulfur molecule (S₈) powder was 8:2 (Example 1), 5:5 (Example 2),and 3:7 (Example 3) was prepared, and then heat-treated for 3 hours in areactor having an inert gas atmosphere in which the internal temperaturewas controlled to 160° C.

The polymer thus obtained will be collectively referred to aspoly(ALA-S). Further, according to the mixing ratio of the rawmaterials, it is represented by poly(ALA:S₈) x:y, where x:y correspondsto the weight ratio of ALA and S₈ in the manufacturing raw materials.Specifically, poly(ALA:S₈) 8:2 is the polymer of Example 1, poly(ALA:S₈)5:5 is the polymer of Example 2, and poly(ALA:S₈) 3:7 is the polymer ofExample 3.

(2) Preparation of a Separator Whose One Surface is Coated withPoly(ALA-S)

Three separators made of polypropylene (pp) material (width: 4.5 cm*length: 4.5 cm* thickness: 25 μm, porosity: 41% by volume) wereprepared.

Each of poly(ALA-S) (Examples 1 to 3) was dissolved in a THF(tetrahydrofuran) solvent to prepare a coating solution, but the solidcontent in 100 wt % of each coating solution was made to be 1 wt %.

Each coating solution was sprayed on one surface of each of theseparators using a manual spray device (Iwata HP-C PLUS) under thecondition of 70° C. Then, it was dried under the condition of 60° C.

Accordingly, the separator coated onto one surface was indicated aspoly(ALA:S8)_x:y_ppx2, and among the latter indications,“poly(ALA:S₈)_x:y” corresponds to the indication method of the polymerused in the coating.

(3) Manufacture of a Battery Including a Separator Whose One Surface isCoated with Poly(ALA-S)

Three types of batteries including the separators of Examples 1 to 3were prepared. In the Experimental Examples described later, a batteryhaving an appropriate shape was selected according to the experimentalmethod.

(3-1) Manufacture of SUS Symmetric Cell

Each separator of Examples 1 to 3 was cut into a circular shape(cross-sectional area: 2.54 cm²), and then placed between two sheets ofstainless steel (SUS) to produce a coin cell, and the electrolyte havingthe composition of 1M LiTFSI DOL/DME (1:1 v/v) was injected into eachcoin cell.

(3-2) Manufacture of Lithium Symmetric Cell

The poly(ALA:S8)_3:7_ppx2 (Example 3) was cut into a circular shape(cross-sectional area: 2.54 cm²), and then placed between two sheets oflithium metal (Li-metal) electrodes to make a lithium symmetriccell-shaped coin, and the electrolyte was injected.

Here, in order to confirm the relationship between the electrolyteadditive and the coated separator, LiNO₃ was added to the composition of1M LiTFSI in DOL:DME=1:1 (v:v), and each lithium symmetric cell wasmanufactured using electrolytes in three cases where the concentrationsof the additive were 0.2 M, 0.5 M, and 0.7 M, respectively.

(3-3) Manufacture of Li/SUS Cell

The poly(ALA:S₈)_3:7_ppx2 (Example 3) was cut into a circular shape(cross-sectional area: 2.54 cm²), and then placed between one lithiummetal (Li-metal) electrode and one stainless steel (SUS) to make alithium half-coin-shaped coin, and the electrolyte was injected. Here,as the electrolyte, LiNO₃ was added to the composition of 1M LiTFSI inDOL:DME=1:1 (v:v), and the concentration of the additive was 0.7 M.

Comparative Example 1

(1) Preparation of Poly(ALA)

Alpha lipoic acid (ALA) powder, one of the raw materials for preparingthe polymer of Examples 1 to 3, was used alone, and heat-treated for 3hours in a reactor having an inert gas atmosphere in which the internaltemperature was controlled to 160° C.

The polymer thus obtained will be referred to as poly(ALA).

(2) Preparation of a Separator Whose One Surface is Coated withPoly(ALA)

Instead of the polymers of Examples 1 to 3, poly(ALA) of ComparativeExample 1 was used to prepare a separator coated onto one surface. Theseparator coated onto one surface was indicated as poly(ALA)_ppx2.

(3) Manufacture of a Battery Including a Separator Whose One Surface isCoated with Poly(ALA)

A battery including the separator of Comparative Example 1 instead ofthe separator of Examples 1 to 3 was manufactured.

Comparative Example 2

In Comparative Example 2, no polymer was prepared.

Instead, one separator of polypropylene (pp) material used in Examples 1to 3 (width: 4.5 cm* length: 4.5 cm* thickness: 25 μm, porosity: 41% byvolume) was prepared, and this separator was indicated by Bare_ppx2.

A battery including the separator of Comparative Example 2 wasmanufactured in the same manner as in Examples 1 to 3.

Comparative Example 3

Also in Comparative Example 3, no polymer was prepared.

Instead, one separator of polypropylene (pp) material used in Examples 1to 3 (width: 4.5 cm* length: 4.5 cm* thickness: 25 μm, porosity: 41% byvolume) was prepared, and this separator was indicated by Bare_ppx2.

On the other hand, Li₂S₈ is also a type of electrolyte additive, and itis known that a sulfide-based film is formed on the surface of a lithiummetal negative electrode in a battery. In this regard, 0.2 M Li₂S₈ wasadded to the composition of 1M LiTFSI in DOL:DME=1:1 (v:v), which wasused as an electrolyte, and Bare ppx2 was used to produce a lithiumsymmetric cell.

Comparative Example 4

In Comparative Example 4, a Li/SUS cell was manufactured using theseparator of Example 3 (poly(ALA:S₈)_3:7_ppx2), but instead of theelectrolyte of Example 3, LiNO₃ as an additive was not added, and theelectrolyte with a composition of 1M LiTFSI in DOL:DME=1:1 (v:v) wasused.

Experimental Example 1: Evaluation of Chemical Formula and Structure ofthe Polymer (Poly (ALA-S)) of Examples 1 to 3

1) Elemental Analysis

The polymers of Examples 1 to 3 (poly(ALA-S)) and the polymer ofComparative Example 1 (poly(ALA)) were each analyzed by an elementalanalyzer (EA), and the results are shown in Table 1 below.

TABLE 1 Raw material mixture ALA:S₈ Elemental analysis result weight offinal material (wt %) Chemical formula of ratio C H O S the finalmaterial Comparative 10:0  46.78 6.76 16.35 30.10C₈H_(13.88)O_(2.10)S_(1.93) Example 1 Example 1 8:2 35.50 4.85 11.0348.62 C₈H_(13.10)O_(1.86)S_(4.11) Example 2 5:5 31.19 4.28 8.40 56.13C₈H_(13.18)O_(1.62)S_(5.40) Example 3 3:7 23.83 3.19 6.89 66.09C₈H_(12.84)O_(1.73)S_(8.32)

According to Table 1, the chemical formulas of the polymers of Examples1 to 3 (poly (ALA-S)) and the polymer of Comparative Example 1 (poly(ALA)) can be determined. It was confirmed that poly(ALA) obtained byheat-treating only ALA (Comparative Example 1) has the chemical formulaof C₈H_(13.88)O_(2.10)S_(1.93), and the sulfur mole fraction thereofcorresponds to a value close to the theoretical value of 2.

On the other hand, in poly(ALA-S) obtained by heat-treating a mixture ofALA and S₈ (Examples 1 to 3), the chemical formulas ofC₈H_(13.10)O_(1.86)S_(4.11) (Example 1), C₈H_(13.18)O_(1.62)S_(5.40)(Example 2), and C₈H_(12.54)O_(1.73)S_(8.32) (Example 3) were confirmed.It was confirmed that the sulfur mole fraction thereof is proportionalto the content of S₈ in the raw material mixture.

2) X-Ray Diffraction Analysis

In addition, the polymers of Examples 1 to 3 (poly(ALA-S)) and thepolymer of Comparative Example 1 (poly(ALA)) were analyzed with adiffraction analyzer using Cu Kα X-ray (X-Ray Diffraction, XRD, RIGAKU),and the results are shown in FIG. 2.

According to FIG. 2, it can be seen that all polymers are amorphous, andthe sulfur element (S) is present in the bulk. From this, inExperimental Example 1, it can be inferred that polymerization wascarried out by a method in which S₈ was bonded to the ring-openedterminal (i.e., sulfur chain) of ALA, regardless of the weight ratio ofALA and S₈.

Experimental Example 2: Surface Observation and Ionic ConductivityEvaluation of the Separator (Poly(ALA:S₈)_x:y_ppx2) of Examples 1 to 3

1) Surface Observation

The separators (poly(ALA:S₈)_x:y_ppx2) of Examples 1 to 3 and theseparator (poly(ALA)_ppx2) of Comparative Example 1 were respectivelyphotographed with a digital camera (Galaxy S9), and shown in FIG. 3a ,and also photographed with a scanning electron microscope (SEM) andshown in FIG. 3 b.

In FIGS. 3a and 3b , it can be seen that both poly(ALA) (ComparativeExample 1) and poly(ALA-S) (Examples 1 to 3) can be uniformly coatedonto one surface of the separator.

2) Ionic Conductivity Analysis

The ionic conductivity of the separators of Examples 1 to 3(poly(ALA:S₈)_x:y_ppx2) and the separator of Comparative Example 1(poly(ALA)_ppx2), ionic conductivity was analyzed using a SUS symmetriccell.

Specifically, for the SUS symmetric cell manufactured including eachseparator, electrochemical impedance spectroscopy was carried out usingan analysis device (VMP3, Bio logic science instrument) under theconditions of amplitude 10 mV and scan range 10 Khz to 100 KHz at 25° C.Based on the impedance analysis result, the ionic conductivity of eachcoated separator was calculated and shown in Table 2 and FIG. 3c below.

For comparison, the uncoated separator (Bare_ppx2) of ComparativeExample 2 was also subjected to impedance analysis in the same manner tocalculate the ionic conductivity, and shown in Table 2 and FIG. 3cbelow.

TABLE 2 Ionic conductivity (S/cm) Example 1 poly(ALA:S₈)_8:2_ppx2)7.60*10⁻⁴ Example 2 poly(ALA:S₈)_5:5_ppx2) 4.13*10⁻⁴ Example 3poly(ALA:S₈)_3:7_ppx2) 3.53*10⁻⁴ Comparative Example 1 (poly(ALA)_ppx2)1.60*10⁻⁴ Comparative Example 2 (Bare ppx2) 9.28*10⁻⁴

According to Tables 2 and FIG. 3c , it is confirmed that as the sulfurcontent in the polymer used for coating the separator increases, theionic conductivity of the coated separator tends to decrease.

In the separators of Examples 1 to 3 (poly(ALA:S₈)_x:y_ppx2),depolymerization of each polymer is performed by a chemical reaction oran electrochemical reaction at the time of being contact with thelithium metal of the negative electrode in the battery, which has beendescribed above.

However, in the case of the SUS symmetric cell used in the ionicconductivity analysis of Experimental Example 2, lithium metal does notexist on the surface of the negative electrode and thus,depolymerization may not occur. Therefore, the ionic conductivityanalyzed in Experimental Example 2 may be intrinsic to each polymer, andas the content of ALA in the polymer increases, swelling due to theelectrolyte increases, and it can exhibit the tendency of ionicconductivity in Table 2.

Experimental Example 3: Manufacture and Evaluation of a LithiumSymmetrical Cell Including the Coated Separator of Experimental Example2

1) Here, in order to confirm the relationship between the electrolyteadditive and the coated separator, a battery made of a lithium symmetriccell including the separator of Example 3 (poly(ALA:S₈)_3:7_ppx2) wasused, but lithium symmetric cell samples having different electrolytecompositions were used.

Specifically, LiNO₃ was added to the composition of 1M LiTFSI inDOL:DME=1:1 (v:v) LiNO₃, but each lithium symmetric cell wasmanufactured using electrolytes in three cases where the concentrationsof the additive were 0.2 M, 0.5 M, and 0.7 M, respectively.

Charging at a current density of 3.0 mA/cm² at 25° C. for 1 hour anddischarging at a current density of 3.0 mA/cm² at 25° C. for 1 hour wasset as one-time charge/discharge cycle, and the results of charging anddischarging each of the lithium symmetric cells are shown in FIG. 4 a.

According to FIG. 4a , it can be seen that the concentration of LiNO₃ isproportional to the lifetime of the lithium symmetric cell. From this,it may be seen that a synergistic effect exists between LiNO₃ andpoly(ALA:S₈)_3:7_ppx2 (Sample 4).

2) In order to more clearly confirm this synergistic effect, 0.7 M ofLiNO₃ was added to the composition of LiTFSI in DOL:DME=1:1 (v:v), andthe electrolyte was controlled, and different types of separators wereapplied.

Specifically, a lithium symmetric cell to which Comparative Example 3(Bare ppx2) and Example 3 (poly(ALA:S₈)_3:7_ppx2) were appliedrespectively as a separator was subjected to charge and discharge, andthe results are shown in FIG. 4b . Here, the charge/discharge conditionsof each lithium symmetrical cell are the same as the charge/dischargeconditions previously performed.

On the other hand, Li₂S₈ is also a type of electrolyte additive, and isknown to form a sulfide-based film on the surface of a lithium metalnegative electrode in a battery. In this regard, 0.2 M of Li₂S₈ wasadded to the composition of 1M LiTFSI in DOL:DME=1:1 (v:v), which wasused as an electrolyte, and Bare ppx2 was used to prepare a lithiumsymmetric cell (Comparative Example 3). The lithium symmetric cell towhich Li₂S₈ of Comparative Example 3 was added was subjected to chargeand discharge under the same conditions, and the results are shown inFIG. 4 c.

According to FIG. 4c , in the case of a lithium symmetric cell in whichno electrolyte additive was introduced without coating the separator, acell life of about 200 cycles (Comparative Example 2) was confirmed, andwhen Li₂S₈ was added to the electrolyte without coating the separator, acell life of about 250 cycles (Comparative Example 3) was confirmed. Onthe other hand, when the separator was coated with poly(ALA-S) and LiNO₃was added to the electrolyte (Example 3), the longest cell life of 400cycles or more was confirmed.

In order to find the cause of the difference in cell lifetime, each ofthe lithium symmetric cells was driven for 10 cycles and then decomposedto recover the lithium metal negative electrode, and the surface of eachlithium metal negative electrode was photographed with a digital camera(FIG. 4d ).

In FIG. 4d , not only in the case of the lithium symmetric cell(Comparative Example 2) in which the electrolyte additive was notintroduced without coating the separator, but also in the case whereLi₂S₈ was added to the electrolyte without coating the separator(Comparative Example 3), black electrolyte decomposition products wereobserved from the surface of the lithium metal negative electrode.

On the other hand, when the separator was coated with poly(ALA-S) andLiNO₃ was added to the electrolyte (Example 3), almost no electrolytedecomposition products were observed from the surface of the lithiummetal negative electrode, and a silver-white lithium surface wasobserved.

In particular, a comparison was made between the case where Li₂S₈ wasadded to the electrolyte without coating the separator (ComparativeExample 3) and the case where the separator was coated with poly(ALA-S)and LiNO₃ was added to the electrolyte (Example 3).

The lithium symmetric cell in each case was driven for 10 cycles asdescribed above, and then decomposed to recover the lithium metalnegative electrode. The surface of each lithium metal negative electrodewas analyzed by X-ray photoelectron spectroscopy (Thermo VG Scientific),and the results are shown in FIGS. 4e and 4 f.

According to FIG. 4e , when Li₂S₈ was added to the electrolyte withoutcoating the separator (Comparative Example 3), peaks such as CF₃, CO₃ ⁻,and O═C═O, which are generally known as electrolyte decompositionproducts, were confirmed.

On the other hand, in FIG. 4f , when the separator was coated withpoly(ALA-S) and LiNO₃ was added to the electrolyte (Example 3), peakssuch as C═O and C—O—C due to the decomposition reaction of the carboxylgroup contained in ALA were confirmed, but no peak due to the otherelectrolyte decomposition products was confirmed. In addition, asulfate-related peak exhibited by the decomposition reaction of LITFSI,a lithium salt in the electrolyte, was also relatively reduced.

From this result, it can be seen that when the separator was coated withpoly(ALA-S) and LiNO₃ was added to the electrolyte, poly(ALA-S) wasdissolved in the electrolyte, and then concentrated on the surface ofthe lithium metal negative electrode to form a protective layer, anddepolymerization of poly(ALA-S) was performed preferentially over thedecomposition of the electrolyte between the protective layer and thelithium metal negative electrode, thereby minimizing electrolytedecomposition and increasing cell life.

Experimental Example 4

In the Experimental Example 4, a battery made of a lithium half cell(Li/SUS) including the separator of Example 3 (poly(ALA:S₈)_3:7_ppx2),and a battery made of lithium half-cell (Li/SUS) including the separatorof Comparative Example 1 (poly(ALA)_ppx2) were compared. Here, as theelectrolyte composition of each battery, those in which 1M LiTFSI wasdissolved in DOL/DME (1:1 v/v), and 0.7M LiNO₃ was added was used.

Specifically, charging at a current density of 2.0 mA/cm² at 25° C. for1 hour and discharging at a current density of 2.0 mA/cm² at 25° C. for1 hour was set as one-time charge/discharge cycle, and each of theLi/SUS cells was charged and discharged for 10 cycles. Thereafter, eachLi/SUS cell was decomposed, and the SEI remaining on the surface of theSUS electrode was analyzed by XPS. Each analysis result is shown in FIG.5a (Example 3) and FIG. 5b (Comparative Example 1).

Looking at both FIG. 5a (Example 3) and 5 b (Comparative Example 1), theseparator of Example 3 (poly(ALA:S₈)_3:7_ppx2) appeared relatively fewpeaks due to electrolyte decomposition such as CF₃, CO₃ ⁻, O═C—OR, C—OR,and SO₄ ²⁻, as compared with the case of applying the separator ofComparative Example 1 (poly(ALA)_ppx2), and this result is in line withExperimental Example 3.

This can be inferred from the result that stable inorganic sulfide suchas Li₂S₂, which is not applied to the separator of Comparative Example 1(poly(ALA)_ppx2), is contained in the SEI in a high ratio.

Meanwhile, the impedance of each of the lithium half cells (Li/SUS) wasanalyzed under the same conditions as in Experimental Example 2.

As a result, in both a battery made of a lithium half cell (Li/SUS)including the separator (poly(ALA:S8)_3:7_ppx2) of Example 3 (FIG. 5c ),and a battery made of a lithium half cell (Li/SUS) including theseparator (poly(ALA)_ppx2) of Comparative Example 1 (FIG. 5d ), it isconfirmed that the R_(film) & R_(et) values are very large in the firstcycle in which the polymers coated on one surface of the separator havenot reacted, but the values gradually decreases as the subsequent cycleprogresses.

Based on EIS (electrochemical impedance spectroscopy) data after 50cycles that the reaction was judged to have completely progressed, whenlooking at the size of the semicircle representing RSEI, it can be seenthat the lithium half cell (Li/SUS) to which the separator(poly(ALA)_ppx2) of Comparative Example 1 is applied is 126Ω, thelithium half cell (Li/SUS) to which the separator(poly(ALA:S₈)_3:7_ppx2) of Example 3 was applied is 60Ω, and in thelatter case, an SEI layer with much higher ionic conductivity is formedon the surface of the negative electrode.

In particular, after 100 cycles, when the separator of ComparativeExample 1 (poly(ALA)_ppx2) was applied, unstable SEI was formed on thesurface of the negative electrode, which appears to cause a sidereaction with the electrolyte to form dead Li, thereby significantlyincreasing the resistance by R_(film)+R_(charge transfer). On the otherhand, when the separator of Example 3 (poly(ALA:S₈)_3:7_ppx2) wasapplied, it is confirmed that SEI with excellent durability is formed,the stable interface is well maintained, and RSEI is kept constant at58Ω without an increase in R_(charge transfer). These results areinferred to be due to the high proportion of inorganic sulfide-based SEI(Li₂S, Li₂S₂) and low electrolyte decomposition products detected in theXPS analysis.

Experimental Example 5

In Experimental Example 5, the difference depending on whether or notLiNO₃ was added as an electrolyte was examined. To this end, a Li/SUScell in which poly(ALA:S₈)_3:7_ppx2 was used as a separator and a 0.7 Mconcentration of LiNO₃ was added to the electrolyte (Example 3), Li/SUScell in which poly(ALA)_ppx2 was used as a separator and LiNO₃ was addedto the electrolyte (Comparative Example 1), and a Li/SUS cell(Comparative Example 4) in which poly(ALA:S₈)_3:7_ppx2 was used as aseparator, but LiNO₃ was not added to the electrolyte, were respectivelydriven under the same conditions as in Experimental Example 4, and theresults are shown in FIG. 6.

In FIG. 6, when defined as the lifetime of a cell until the coulombicefficiency drops below 80%, the lifetime of a Li/SUS cell (Example 3) inwhich poly(ALA:S₈)_3:7_ppx2 was used as a separator and LiNO₃ with aconcentration of 0.7 M was added to the electrolyte was confirmed to beabout 140 cycles. This appears to be due to the result that not only aninorganic sulfide-based compound is produced as a polymer reductiondecomposition product of poly(ALA:S8)_3:7_ppx2, but also an inorganicnitride compound, an inorganic nitrate compound, or a mixture thereof isproduced as the reductive decomposition product of LiNO₃.

On the other hand, it was confirmed that the Li/SUS cell (ComparativeExample 1) in which poly(ALA)_ppx2 was used as a separator and LiNO₃ wasadded to the electrolyte has reached the end of its life after beingdriven unstable up to 100 cycles. On the other hand, it is confirmedthat the Li/SUS cell (Comparative Example 4) in whichpoly(ALA:S₈)_3:7_ppx2 is used as a separator but LiNO₃ is not added tothe electrolyte is immobilized by SEI containing only inorganic sulfideas an inorganic compound, and reaches the end of its life after only 20cycles.

1. A lithium metal negative electrode comprising: a negative electrode;and a protective layer present on the negative electrode, wherein theprotective layer comprises: a polymer of alpha lipoic acid (ALA) andsulfur molecule (S₈), a depolymerized product of the polymer, aninorganic sulfide-based compound, and at least one of an inorganicnitride-based compound or an inorganic nitrate-based compound.
 2. Thelithium metal negative electrode according to claim 1, wherein theinorganic sulfide-based compound comprises at least one of Li₂S orLi₂S₂.
 3. The lithium metal negative electrode according to claim 1,wherein the inorganic nitride-based compound comprises Li₃N, and theinorganic nitrate-based compound comprises LiN_(x)O_(y) wherein x=1 or2, and y=2 or
 3. 4. The lithium metal negative electrode according toclaim 1, wherein the polymer is obtained by polymerizing alpha lipoicacid and sulfur molecules in a weight ratio of 10:1 to 1:10.
 5. Thelithium metal negative electrode according to claim 1, wherein thepolymer of alpha lipoic acid (ALA) and sulfur molecule (S₈) isrepresented by the following Chemical Formula 1:

in the Chemical Formula 1, R₁ to R₄ are each independently a substitutedor unsubstituted C1 to C3 alkylene group; 3≤n≤10; and 1<x≤10.
 6. Thelithium metal negative electrode according to claim 1, wherein thedepolymerized product of the polymer is an oligomer represented by atleast one of the following Chemical Formula 1-1 or a monomer representedby the following Chemical Formula 1-2:

in the Chemical Formula 1-1, R₁ to R₄ are each independently asubstituted or unsubstituted C1 to C3 alkylene group; 1≤y≤6; and 1≤z≤5,

in the Chemical Formula 1-2, R₁ to R₄ are each independently asubstituted or unsubstituted C1 to C3 alkylene group.
 7. The lithiummetal negative electrode according to claim 6, wherein the oligomer andthe monomer each independently further comprise a lithium cation.
 8. Thelithium metal negative electrode according to claim 1, wherein thenegative electrode is a lithium free anode consisting of only a coppercurrent collector; or the negative electrode comprises: a copper currentcollector and a lithium metal layer present on the copper currentcollector.
 9. A lithium metal battery comprising: the lithium metalnegative electrode of claim 1; a positive electrode; a separator locatedbetween the lithium metal negative electrode and the positive electrode;and an electrolyte impregnated in the separator, wherein the electrolytecomprises a lithium salt, an organic solvent, and LiNO₃.
 10. The lithiummetal battery according to claim 9, further comprising a coating layercomprising the polymer is present on at least one surface facing thenegative electrode among both surfaces of the separator facing thenegative electrode.
 11. The lithium metal battery according to claim 9,wherein a concentration of the additive in the electrolyte is 0.1 M to1.0 M.
 12. The lithium metal battery according to claim 9, wherein thelithium salt comprises lithium bis(trifluoromethanesulfonyl)imide(LiTFSI).
 13. The lithium metal battery according to claim 9, whereinthe organic solvent is a mixture of 1,3-dioxolane (DOL) anddimethoxyethane (DME) in a volume ratio of 3:7 to 7:3.
 14. A method forproducing a lithium metal battery comprising the steps of: preparing apolymer of alpha lipoic acid (ALA) and sulfur molecule (S₈); coating thepolymer onto one surface of a separator; preparing an electrode assemblywherein the separator surface coated with the polymer is positionedopposite to a lithium metal negative electrode, and the other surface ofthe separator is positioned opposite to a positive electrode, injectingan electrolyte into the separator in the electrode assembly; andpackaging the assembly after injecting the electrolyte to obtain thelithium metal battery.
 15. The method for producing a lithium metalbattery according to claim 14, wherein in the obtained lithium metalbattery, a part of the polymer coated on the surface of the separator isdepolymerized by reacting the polymer with the lithium metal of thenegative electrode, another part of the polymer coated on the surface ofthe separator is reduced and decomposed by reacting with theelectrolyte, and wherein the electrolyte comprises LiNO₃ and the LiNO₃is reduced and decomposed by reacting with the lithium metal of thenegative electrode.
 16. The method for producing a lithium metal batteryaccording to claim 14, wherein applying heat at a temperature in a rangeof 150° C. to 175° C. to the mixture of the alpha lipoic acid (ALA) andthe sulfur molecule (S₈) in the step of preparing a polymer.